I don't know if the normal illumination colour is green but if the cluster bulbs have been replaced with LED versions by a previous owner that could explain them not dimming. LEDs dim but only slightly.
FSM states...
CLUSTER ILLUMINATION
The EMIC has several illumination lamps that are illuminated whenever the exterior lighting is turned
On. The illumination intensity of these bulbs and of the vacuum-fluorescent electronic display are controlled
by the instrument cluster microprocessor based upon dimming messages received from the
Body Control Module (BCM) over the PCI data bus.
The BCM uses inputs from the headlamp and panel dimmer switches on the left (lighting) multi-function
switch control stalk and internal programming to decide what dimming message is required. The BCM
then sends the proper dimming messages to the EMIC over the PCI data bus. The illumination lamps
receive battery current through the EMIC electronic circuit board, and are grounded at all times. The
BCM also has several hard wired panel lamp driver outputs and sends the proper panel lamps dimming
level messages over the PCI data bus to coordinate the illumination intensity of all of the instrument
panel lighting and the vacuum fluorescent displays of other electronic modules on the PCI data bus. Vehicles
equipped with the Auto Headlamps option have an automatic parade mode. In this mode, the BCM
uses an input from the auto headlamp light sensor to determine the ambient light levels. If the BCM
decides that the exterior lighting is turned on in the daylight, it overrides the selected panel dimmer
switch signal by sending a message over the PCI data bus to illuminate all vacuum fluorescent displays
at full brightness for easier visibility in daytime light levels. The automatic parade mode has no
effect on the incandescent bulb illumination intensity.
Proper testing of the cluster illumination lamps, the VFD dimming level, the BCM, and the PCI data
bus dimming level messages requires the use of a DRBIII scan tool. Refer to the appropriate diagnostic
information. Each of the cluster illumination lamps is located on the instrument cluster circuit
board. Each cluster illumination lamp has a replaceable bulb and bulb holder.